The present invention relates to an offset printer, and more particularly, to a multiple color printer provided with a sheet feed mechanism for feeding a sheet or other print medium to a sheet feed cylinder.
A multiple color offset printers are known for printing in multiple colors. For example, Japanese Patent Application Publication Toku-Hyou-Hei-9-510410 discloses a four color offset printer in which a single impression cylinder, a single sheet discharge mechanism, a single sheet feed conveyor (feeder), a single sheet feed cylinder, two blanket cylinders, two plate cylinders, and ink roller groups for four colors are provided.
A sheet feed mechanism including the sheet feed conveyor and the sheet feed cylinder is adapted for supplying a sheet to a surface of the impression cylinder. The impression cylinder has an outer peripheral surface where the sheet is held. The sheet discharge mechanism is adapted to remove the sheet from the surface of the impression cylinder. The blanket cylinders are adapted to press the sheet against the impression cylinder to form an ink image on the sheet.
The outer peripheral surface of the impression cylinder is provided with grippers, each gripper having a pawl at its tip for holding a sheet. The impression cylinder is equally divided into three segments for holding each sheet at each segment, and is driven by a drive motor. Axes of the blanket cylinders and the paper feed cylinder are disposed in parallel with the axis of the impression cylinder, and outer peripheral surfaces of the blanket cylinders and the paper feed cylinder are in contact with the outer peripheral surface of the impression cylinder. The blanket cylinders, the sheet feed conveyer, the sheet feed cylinder, and the sheet discharge mechanism are rotated by rotation of the impression cylinder.
Each plate cylinder is provided with a plate at its outer peripheral surface. The axes of the two plate cylinders are disposed in parallel with the axes of the two blanket cylinders, and each plate cylinder is in contact with a corresponding blanket cylinder, so that each plate cylinder is rotated by the rotation of the corresponding blanket cylinder. The outer peripheral surface of each plate cylinder is equally divided into two plate segments, and each plate segment has a peripheral length equal to that of each segment of the impression cylinder. Each plate segment is provided with a plate for one specific color, and a different color is associated with each plate. Therefore, totally four plates for four different colors are provided in the two plate cylinders.
Each ink roller group is adapted for supplying ink to the plate on the plate cylinder. Two groups of ink rollers are provided for one plate cylinder so as to supply inks of two colors. Therefore, totally four groups of ink rollers are provided for supplying inks of four different colors. The ink rollers have axes in parallel with the axis of the plate cylinder, and are in contact with the plate cylinder. The ink rollers are rotated by the rotation of the plate cylinder.
In this way, in the offset printer capable of performing four color printing with the two blanket cylinders, each sheet is printed with two colors during each single rotation of the impression cylinder, and printing of the additional two color is performed during the second rotation of the impression cylinder. That is, each sheet is held on the impression cylinder for two rotations thereof, and thereafter must be released from the impression cylinder by the sheet discharge mechanism. If each sheet is supplied to the impression cylinder at every single rotation of the impression cylinder, it becomes impossible to perform four color printing with respect to each sheet. To avoid this, a sheet is supplied to every other segment of the impression cylinder. For example, if a sheet is supplied to a first segment, then, a sheet is not supplied to a second segment, but a sheet is supplied to a third segment. The supplied sheet is held on the impression cylinder until completion of two rotations thereof and is then discharged from the impression cylinder by the sheet discharge mechanism.
No specific arrangement is proposed in the Japanese patent application publication No. Toku-Hyou-Hei 9-510410 for supplying a sheet to every other segment of the impression cylinder.
One conceivable arrangement is to design the outer peripheral length of the sheet feed cylinder equal to the peripheral length of each segment of the impression cylinder, and supply a sheet to the impression cylinder at every second rotation of the sheet feed cylinder. To this effect, it is necessary to supply sheet to the sheet feed roller every second rotation thereof. Because a single sheet must be transferred to the impression cylinder at every second rotation of the sheet feed cylinder, if a sheet is supplied to the sheet feed roller at every single rotation thereof, two sheets must be held at the outer surface of the sheet feed roller, or else a first sheet already held at the sheet feed cylinder must be released therefrom when the sheet feed roller receives a subsequent sheet. In the latter case, the sheet supplying system may operate abnormally.
In another aspect, when the sheet is to be supplied to the impression cylinder, orientation of the sheet and lateral position of the sheet are important factors. To this effect, a sheet abutment mechanism (a front lay mechanism) for adjusting the orientation of the sheet and a sheet lateral position control mechanism for adjusting the lateral position of the sheet are conventionally provided at positions upstream of the sheet feed cylinder. However, no detailed arrangement has been proposed for driving the sheet abutment mechanism and the sheet lateral position control mechanism in order to supply sheet to the impression cylinder at every second rotation of the sheet feed cylinder.
It is therefore an object of the present invention to provide a multiple color offset printer having a sheet feed mechanism capable of supplying each sheet from the sheet feed cylinder to the impression cylinder at a desired timing.
This and other objects of the present invention will be attained by an improved multiple color offset printer for forming a multiple color image on a sheet including a plate cylinder, a blanket cylinder, an impression cylinder, a sheet feed cylinder, a sheet feed cylinder gripper, a feeder board, and a swing mechanism. The plate cylinder is rotatable about its axis and has an outer peripheral surface equally divided in a circumferential direction into at least two color regions. An ink image is formed on each color region, and each color region has an equal circumferential length. The blanket cylinder is rotatable about its axis and has an outer peripheral surface in contact with the color regions and equally divided in a circumferential direction into at least two color sections. Each ink image on each color region is transferred to each color section, and each color section has an equal circumferential length. The impression cylinder is rotatable about its axis and has an outer peripheral surface on which the sheet is held and transferred. Each color section is in contact with the outer peripheral surface of the impression cylinder for transferring each ink image on each color section onto the sheet. The outer peripheral surface of the impression cylinder is equally divided into a plurality of segments each having peripheral length equal to the circumferential length of the color region and the color section. The sheet feed cylinder is rotatable about its axis and has an outer peripheral surface on which a sheet is held and transferred and in contact with the outer peripheral surface of the impression cylinder for transferring the sheet to the impression cylinder. The outer peripheral surface of the sheet feed cylinder has a peripheral length equal to each peripheral length of each segment of the impression cylinder. The sheet feed cylinder gripper is provided at the outer peripheral surface of the sheet feed cylinder and is movable along with the rotation of the sheet feed cylinder. The feeder board is in a form of a belt conveyor for feeding a sheet to the sheet feed cylinder. The swing mechanism is provided adjacent the sheet feed cylinder and is moveable to a sheet transferring position where the sheet is transferred from the feeder board to the sheet feed cylinder gripper, when the sheet feed cylinder gripper reaches the sheet transferring position. The swing mechanism provides a cyclic period for transferring the sheet from the feeder board to the sheet feed cylinder gripper, the cyclic period being substantially equal to a rotation period of a plurality of times of rotation of the sheet feed cylinder, the plurality of times being equal to the number of color regions of the plate cylinder.
A sheet transfer passage is provided between the feeder board and the swing mechanism, and preferably, the printer further includes a front lay mechanism provided between the feeder board and the swing mechanism. The front lay mechanism includes an abutment member, and an abutment member driving mechanism. The abutment member is movable between a retracted position away from the sheet transfer passage for allowing the sheet to pass through the abutment member and a projecting position projecting into the sheet transfer passage for abutting a leading end of the sheet against the abutment member to temporarily preventing the sheet from being transferred from the feeder board to the swing mechanism and to align the leading end of the sheet in parallel with the axis of the sheet feed cylinder making use of sheet feeding force of the feeder board. The abutment member driving mechanism is adapted for driving the abutment member to move between the projecting position and the retracted position. The abutment member driving mechanism drives the abutment member from the projecting position to the retracted position in synchronism with the above described rotation period.
The sheet transfer passage has a width in a widthwise direction of the sheet, and preferably, the printer further includes a sheet lateral position control mechanism for moving the sheet in its widthwise direction to a predetermined position on the sheet transfer passage. The sheet lateral position control mechanism includes a sheet holding portion and a sheet holding portion driving mechanism. The sheet holding portion is movable in the widthwise direction of the sheet on the sheet transfer passage between the feeder board and the abutment member. The sheet holding portion driving mechanism is adapted for driving the sheet holding portion in synchronism with the rotation period.
In another aspect of the invention, there is provided a multiple color offset printer for forming a multiple color image on a sheet including the plate cylinder, the blanket cylinder, the impression cylinder, the sheet feed cylinder, the sheet feed cylinder gripper, the feeder board, the swing mechanism, and a front lay mechanism. The front lay mechanism is provided between the feeder board and the swing mechanism for regulating a leading end position of the sheet. The sheet transfer passage is provided between the feeder board and the swing mechanism. The front lay mechanism includes and abutment member and an abutment member driving mechanism. The abutment member is movable between a retracted position away from the sheet transfer passage for allowing the sheet to pass through the abutment member and a projecting position projecting into the sheet transfer passage for abutting the leading end of the sheet against the abutment member to temporarily preventing the sheet from being transferred from the feeder board to the swing mechanism and to align the leading end of the sheet in parallel with the axis of the sheet feed cylinder making use of sheet feeding force of the feeder board. The abutment member driving mechanism is adapted for driving the abutment member to move between the projecting position and the retracted position. The abutment member driving mechanism drives the abutment member from the projecting position to the retracted position in synchronism with a rotation period of a plurality of times of rotation of the sheet feed cylinder, the plurality of times being equal to the number of color regions of the plate cylinder.
With the configuration described above, the time period required for the feeder board to transport a single sheet to the sheet feed cylinder is equal to the time period required for the sheet feed cylinder to rotate in a number of times equivalent to the number of color sections on the blanket cylinder. Therefore, a single sheet can be supplied from the feeder board to the sheet feed cylinder each time the sheet feed cylinder rotates a number of times equivalent to the number of the color sections on the blanket cylinder. Thus, a sheet can be supplied to the sheet feed cylinder at a timing that matches supply of a single sheet from the sheet feed cylinder to the impression cylinder, so that the sheet transfer is smoothly accomplished.
Further, the abutment member driving mechanism drives the abutment member from the projecting position where the abutment member intrudes into the sheet transfer passage between the feeder board and the swing mechanism into the retracted position separated from the sheet transfer passage, in synchronization with the time period required for the sheet feed cylinder to rotate in a number of times equivalent to the number of color sections on the blanket cylinder. Sheets can be supplied from the feeder board to the sheet feed cylinder only when the abutment member is in the retracted position. Accordingly, even if a sheet somehow was transported to the feeder board at an inappropriate timing, the sheet can be supplied to the sheet feed cylinder at a timing that matches supply of a single sheet from the sheet feed cylinder to the impression cylinder as long as the abutment member drive portion moves the abutment member from the projecting position to the retraction position at an appropriate timing. As a result, sheet transfer operation can be smoothly performed.
Furthermore, the sheet holding portion drive mechanism moves the sheet holding portion in synchronization with the time period required to rotate the sheet feed cylinder a number of times equivalent to the number of color sections on each blanket cylinder. Accordingly, sheet positioning operations can be performed in synchronization with the time required to rotate the sheet feed cylinder in a number of times equivalent to a number of color sections on each blanket cylinder. Therefore, wasteful drive operation of the sheet lateral position control mechanism can be dispensed with. Also, is avoidable an improper sheet transport caused by driving the sheet lateral position control mechanism at improper position of the sheet relative to the sheet holding portion.